In systems such as steam turbines and the like, it is necessary to measure temperature at conditions of high pressure and temperature for monitoring and controlling of such systems. When the temperature information from a temperature sensor is utilized to measure or control a process or operation of a system or equipment, it is essential in many cases that the temperature sensitive device have a fast response, for example to prevent overshoot and instability. When a thermcouple is used as a temperature sensor, it is common to isolate the juntion from the working fluid by means of a thermocouple well. Changes in temperature must communicate to the thermocouple junction through the walls of the thermocouple well and a delay in response occurs. Thus, such construction is not generally satisfactory when fast response is required.
There have been attempts in the prior art to improve the response of thermocouples; for example, see U.S. Pat. No. 4,460,802 to Benedict et al. This patent discloses several variations in design but basically teaches a flat disc of one metal radially welded to a tubular element of dissimilar metal so as to form a well-likd structure. The leads are attached to the disc element and the tubular element with the radial junction therebetween producing the desired electromotive fore when the temperature at the junction differs from that of the external reference junction of the thermocouple system. An improved speed of response is claimed over the conventional well housing a small junction since the dissimilar metals and the junction are exposed directly to the fluid being monitored.
While an improvement in response may be obtained from the Benedict et al thermocouple, it has certain disadvantages when used in systems in which the fluids are under high pressure. Pressure on the disc element will create stresses in the radial junction which may result in cracking or separation. As will be understood, such pressure will tend to cause the disc element to dish and therefore will tend to separate the junction at the periphery of the disc. Thus, the radial arrangement may not be entirely suitable for use in high pressure systems.
Therefore, there is a need for a thermocouple element for use in high-pressure, high-temperature applications hving a very fast response yet which will be free from radial and other excessive stresses at the junctions of the dissimilar metals.